Set the edge of the grout saw blade onto the surface of the grout. Dig into the grout, holding the saw with both hands and pressing firmly. Push the blade forward and pull it back, breaking up the surface.
Continue scraping with the grout saw all along the grout line, breaking up the grout and digging it out. Get down into the grout line by at least about ¼ inch all along the line. Repeat for each line. Take care not to scrape the "stone" façade.
Put grout mix and water in a bucket in the ratio specified on the grout package. Make about half a gallon for a mid-sized wall. Mix the grout and water with a drill and mixing bar.
Mix the grout five minutes, to the consistency of cake frosting. Let it sit in the bucket for 10 minutes. Stir it for another three minutes.
Open the back end of your grout bag. Scoop the grout from the bucket to bag, using your wooden spoon. Close up the bag.
Set the tip of the grout bag at one end of the first open grout line between the fake stones. Compress the bag with both hands, pushing out the grout into the line.
Drag the bag backward along the open line while continuing to compress it, filling the line with grout. Follow the turns of the line to the end. Wipe away any grout that gets on the stones, using a damp sponge .
Run the back of the wood spoon over the surface of the new grout, smoothing it out. Repeat for each open line. Let the grout set for 24 hours.